1. Field of the Invention
The present invention relates to gas leak detection methods for fuel cells, and particularly relates to a gas leak detection method for leaks due to damage or deterioration of solid polymer electrolyte membranes.
2. Description of Related Art
For a fuel cell which is carried in a fuel cell vehicle, for example, there is a fuel cell in which a solid polymer electrolyte membrane made from solid polymer ion exchange membrane is sandwiched between an anode and a cathode horizontally, and sandwiching outside of the anode and cathode is a pair of separators to form a fuel cell unit, and a stack is formed by stacking a plurality of fuel cell units each of which is provided with a hydrogen gas flow passage to which a hydrogen gas is supplied as a fuel gas, an air flow passage to which air including oxygen as an oxidizing gas is supplied, and a coolant liquid flow passage to which coolant liquid is supplied. Hereinafter, a fuel gas and an oxidizing gas may be called xe2x80x9creaction gasesxe2x80x9d in generally. In a fuel cell, a hydrogen ion which is generated in an anode by a catalytic reaction moves to a cathode after passing a solid polymer electrolyte membrane and generates electric current by having electrochemical reaction with oxygen in a cathode. After that, heat which is generated during electric current generation is removed by a coolant liquid in a coolant liquid flow passage so as to cool down a fuel cell.
In this fuel cell, the solid polymer electrolyte membrane has a function of allowing permeation of a hydrogen ions as an electrolyte, and a function to act as a partition wall so as to separate a hydrogen gas in a hydrogen gas flow passage and an oxidizing gas (air) in an air flow passage. Therefore, if this solid polymer electrolyte membrane has small holes, a hydrogen gas in a hydrogen gas flow passage leaks to an air flow passage.
Also, a hydrogen has high permeability, and even in a solid polymer electrolyte membrane which does not have a small hole, a hydrogen permeates (gas cross leak) a solid polymer electrolyte membrane in a molecular form. When a thickness of a solid polymer electrolyte membrane becomes reduced due to change over time, the amount of hydrogen permeation increases.
Here, as a root cause of thinning and damage of a solid polymer electrolyte membrane, damage to a fuel cells due to operating conditions for generating electric current such as low humidity, high temperature, and high work load, and damage caused by dynamic influences such as pressure differences between an anode and a cathode can be named.
In this way, when a hydrogen gas leaks to an air flow passage in a fuel cell, a hydrogen reacts with oxygen in the air in the air flow passage and heat is generated because hydrogen is a combustible gas; thus, there is a concern that a fuel cell may be adversely affected. Therefore, in a fuel cell, it is necessary to find leaks of hydrogen due to damage to a membrane or a thinning of solid polymer electrolyte membrane so as to deal with such a situation by replacing the solid polymer electrolyte membrane.
Conventionally, a method for a gas leak detection method in a fuel cell has been suggested as follows.
In a gas leak detection method which is disclosed in a Published Japanese Translation No. 2000-513134 of a PCT International Publication, an anode and a cathode of a fuel cell are purged by a nitride gas, a nitride gas is exhausted, and furthermore hydrogen is supplied to an anode while oxygen is supplied to a cathode, and the pressure of the gas at the cathode is made greater than the pressure of the gas at the anode keeping a reaction gas flow passage. In this state, an output voltage without electric current flow passages being closed such as an open circuit voltage (OCV) is measured so as to detect the gas leak by utilizing a detecting principle that an OCV decreases rapidly when a gas leak exists.
Also, in a gas leak detection method which is disclosed in a Japanese Unexamined Patent Application, First Publication No. Hei 5-205762, the existence of gas leak and the size of the gas leak are detected by closing a reaction gas flow passage of a fuel cell, supplying a nitrogen gas to an anode of the closed flow passage so as to generate a predetermined pressure difference between an anode and a cathode, and measuring the amount of nitrogen gas which leaks to a cathode.
Also, in a gas leak detection method which is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 9-27336, an anode and a cathode of a fuel cell are purged by a nitride gas, an electric current to a fuel cell is stopped, and furthermore hydrogen is supplied to an anode while an oxygen containing gas is supplied to a cathode, output voltage of a fuel cell is measured while reducing the supply amount of an oxygen containing gas to a cathode gradually so as to detect the gas leak by utilizing a detecting principle that the output voltage decreases rapidly when a gas leak exists.
Furthermore, in a method and apparatus for detecting a leak within a fuel cell which is disclosed in International Publication No. WO 00/39870, a gas leak in a fuel cell and the existence of a gas leak from an electrolyte membrane are detected by supplying a tracer gas which is an inert gas such as helium, argon, carbon dioxide, and nitrogen to one reaction gas which is supplied to a fuel cell, and measuring the concentration of tracer gas in another reaction gas.
Also, in a method and apparatus for detecting a gas leak which is disclosed in International Publication No. WO 98/13890, a gas leak is detected by detecting heat with an infrared ray camera caused by a reaction between an oxygen and a hydrogen due to a gas leak.
In a gas leak detection method which is disclosed in Published Japanese Translation No. 2000-513134 of a PCT International Publication, a gas leak is detected by making a pressure of gas in a cathode greater than a pressure of gas in an anode so as to monitor OCV when an oxygen in a cathode permeates a solid polymer electrolyte membrane and reaches an anode. However, an oxygen molecule is too big to permeate a solid polymer electrolyte membrane. Therefore, the OCV dispersion is small, and it was a problem in that detecting accuracy is not reliable. Also, in such a detection method, OCV change is monitored while closing a reaction gas flow passage. However, it is difficult to distribute a reaction gas uniformly in each cell in a stack. Therefore, the OCV dispersion is large, and there was a disadvantage in that the gas leak detecting accuracy became unreliable.
Also, in a gas leak detection method which is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 5-205762, a sensor which detects a nitrogen gas is necessary; thus, there was a problem in that the entire system became complicated. Furthermore, in such a detection method, although it is possible to detect a gas leak when a solid polymer electrolyte membrane has small holes, it is difficult to detect an increase of amount of gas permeation (gas leak) due to thinning of a membrane.
Also, in a gas leak detection method which is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 9-27336, an output voltage of a fuel cell is measured while reducing a supplying amount of an oxygen containing gas to a cathode gradually. However, there was a problem in that the detecting accuracy is unreliable because variance of the output voltage according to a change of the supplying amount of an oxygen containing gas is small.
In a method and apparatus for detecting a leak within a fuel cell which is disclosed in International Publication No. WO 00/39870, a tracer gas supplying device is necessary, and a sensor for measuring density of a tracer gas is also necessary. Therefore, there was a problem in that the overall system became complicated. Also, in such a detection method, although it is possible to detect a gas leak when a solid polymer electrolyte membrane has small holes, it is difficult to detect an increase of gas permeation (gas leak) amount due to the thinning of a membrane.
Also, in a method and apparatus for detecting a gas leak which is disclosed in International Publication No. WO 98/13890, there was a problem in detecting a gas leak while a fuel cell is disposed as a built-in stack because it is necessary to detect heat directly on a surface of a solid polymer electrolyte membrane.
Here, an object of the present invention is to provide a gas leak detection method in which a gas leak in a fuel cell can be detected easily and very accurately.
In order to solve the above-mentioned problems, a first aspect of the present invention is characterized in that, in a gas leak detection method for a fuel cell (for example a fuel cell unit 55 or a fuel cell 1 which are mentioned later in an embodiment) having a solid polymer electrolyte membrane (for example, a solid polymer electrolyte membrane 51 which is mentioned later in an embodiment) is sandwiched between an anode (for example, an anode 52 which is mentioned later in an embodiment) and a cathode (for example, a cathode 53 which is mentioned later in an embodiment), output voltage is measured in an activated over-potential region in a state in which pressure of fuel gas (for example, a hydrogen gas which is mentioned later in an embodiment) which is supplied to the anode is maintained higher than pressure of gas (for example, an air which is mentioned later in an embodiment) which is supplied to the cathode, and a gas leak is determined to exist when the output voltage is lower than a predetermined voltage value.
By such a gas leak detection method, when a solid polymer electrolyte membrane has small holes or becomes thin, when the pressure at an anode is higher than the pressure at a cathode, the amount of fuel gas which permeates a solid polymer electrolyte membrane from an anode to a cathode increases. As a result, an output voltage in an activated over-potential region decreases due to the existence of a fuel gas molecule near an electrode of a cathode. Here, a voltage dropping ratio is larger than the increasing ratio of fuel gas permeation in an activated over-potential region; thus, the dropping amount by which output voltage drops quite large; therefore, a gas leak can be detected quite sensitively. Accordingly, it is possible to determine that a gas is leaking when the output voltage is lower than a predetermined voltage value.
A second aspect of the present invention is characterized in that a gas leak detection method for a fuel cell of which a solid polymer electrolyte membrane is sandwiched between an anode and a cathode has steps in which pressure of fuel gas which is supplied to an anode is maintained to be higher than pressure of gas which is supplied to a cathode by changing either of pressure of fuel gas which is supplied to the anode and pressure of gas which is supplied to the cathode in a pulse manner, output voltage is measured in an activated over-potential region, and a gas leak is determined to exist when the output voltage is lower than a predetermined voltage value. Here, the same structures and reference numerals indicate the above-mentioned aspects of the present invention as in the case of the first aspect of the present invention.
By such a gas leak detection method in a second aspect of the present invention, comparing to an effect of the first aspect of the present invention, a pressure at an anode is made higher than a pressure at a cathode in a pulsed manner. Therefore, output voltage of an activated over-potential region drops in a pulsed manner. The rest of the effect obtained in the second aspect is the same as in the case of the first aspect.
A third aspect of the present invention, in a first or a second aspect of the present invention, is characterized in that it is performed while passing gas through an anode and a cathode.
By such a gas leak detection method, gas leak detection can be performed while supplying a gas to an anode and a cathode stably; thus, an output voltage can be stable.
As above explained, according to a first aspect of the present invention, it is possible to determine that a gas leak has occurred when an output voltage which is measured in an activated over-potential region is lower than a predetermined voltage value while maintaining pressure of a fuel gas which is supplied to an anode higher than the pressure of a gas which is supplied to a cathode. Therefore, it is possible to detect a gas leak easily. Also, an effect in that detecting accuracy is improved can be realized.
According to a second aspect of the present invention, it is possible to determine that a gas leak has occurred if an output voltage which is measured in an activated over-potential region while pressure of a fuel gas which is supplied to an anode is relatively higher than pressure of a gas which is supplied to a cathode is lower than a predetermined voltage value. Therefore, it is possible to detect a gas leak easily. Also, an effect in that detecting accuracy is improved can be realized. Also, when pressure of a gas at an anode is higher in a pulsed manner than pressure of a gas at a cathode, an output voltage follows a pressure variance and rapidly drops in a pulsed manner. Therefore, an effect in that detecting accuracy further is improved can be realized.
According to a third aspect of the present invention, it is possible to perform a gas leak detection while supplying a gas to an anode and a cathode stably. Therefore, an output voltage becomes stable, and detecting accuracy is improved.